1. Field of the Invention
The present invention relates to the field of generating terahertz radiation. More specifically, the invention pertains to terahertz radiation generation using a phase matched optical rectification technique.
2. Description of the Related Technology
Terahertz radiation is an electromagnetic wave with a frequency on the order of one trillion cycles per second. It is potentially useful for many detection applications because of the fact that many materials that are opaque when viewed with visible light, become transparent when viewed using terahertz radiation. For example, a terahertz image can be employed to reveal weapons concealed under clothing. Although x-rays can also be used for this purpose, terahertz radiation has the advantage that it is non-ionizing and therefore it is expected to be relatively harmless to human tissue.
Terahertz radiation is also of interest for use in spectroscopic applications due to the fact that many molecules have spectroscopic signatures in the terahertz frequency range. Terahertz radiation also has applications in communications, electronic material characterization and the development of high-speed optoelectronic devices. Terahertz radiation systems can also be used to monitor public facilities and detect toxic chemicals and biologic agents, as well as trace explosives in a continuous autonomous manner.
There are several approaches to the generation of terahertz radiation. One approach involves the use of photoconductors. Another approach employs nonlinear optical frequency conversion techniques. In the nonlinear optical frequency conversion approach, second or higher order nonlinear effects in unbiased materials are used. One useful technique is the optical rectification approach which is simpler than the photoconductive approach since no electrical bias is required.
A variety of different electro-optic materials have been proposed for terahertz optical rectification media. U.S. Pat. No. 5,543,690 discusses a number of these materials. U.S. Pat. No. 5,543,690 also describes an apparatus for generating high energy terahertz radiation including a laser effective to produce subpicosecond optical pulses and a mosaic comprising a plurality of planar electro-optic crystals fastened together edge to edge. Each of the crystals is oriented so that its molecular dipole axis is oriented in the form of a grid in optical communication with the subpicosecond optical pulses. The plurality of crystals are said to behave as a single large electro-optic crystal to produce high energy terahertz radiation by optical rectification.
One of the primary problems to the utilization of terahertz radiation in practical applications is the lack of efficient and convenient terahertz radiation sources. One physical process which has been exploited to generate terahertz radiation is photonic downconversion. Photonic downconversion is a set of methods whereby laser radiation is converted to radiation of a lower frequency as it passes through a crystal.
Gallium selenide has been identified as a highly nonlinear, low loss crystal with favorable phase matching characteristics. Ding, Y. and Zotova, I., “Second-order nonlinear optical materials for efficient generation and amplification of temporally-coherent and narrow-linewidth terahertz waves,” Optical and Quantum Electronics 32, pp. 531-552 (2000). Power conversion efficiencies of 10−4 have been reported using such gallium selenide crystals. Shi, W., et al., “Efficient, tunable, and coherent 0.18-5.27 THz source based on GaSe crystal,” Optics Lett. 27, pp. 1454-1456 (2002). However, these experiments required elaborate optical systems and produced very little terahertz power.
One problem with gallium selenide downconversion is that it is difficult to sustain the phase matching condition over a sufficient distance to generate significant amounts of terahertz power. This problem is further complicated by the fact that the mechanical properties of gallium selenide crystals precludes the possibility of cutting or polishing the crystal. Also, efficient coupling of the pump radiation into the gallium selenide crystal at the correct angle and efficient coupling of the terahertz radiation from the gallium selenide crystal to the environment both present difficulties.
Although the gallium selenide crystals cannot be cut or polished, these crystals can be cleaved along planes orthogonal to the internal “crystal axis” to produce a polished surface. Nevertheless, the inability to machine the gallium selenide crystals creates a problem in terms of coupling into the correct phase matching angle. In particular, the index of refraction for 800 nm pump radiation in gallium selenide is about η0=2.85. Using Snell's law, one finds that the largest possible angle of propagation with respect to the crystal axis (corresponding to an angle of incidence of 90°) is about 20.5°. This is significantly less than the phase matching angle of about 28° needed to generate radiation with a 300 μm wavelength. Further, the reflection losses at the surface would be unacceptably high for such large angles of incidence.
The phase matching angle can be calculated using the formula: nθ(ωs)=ng(ω0). In other words, the phase velocity of the THz radiation must equal the group velocity of the laser pulse. This condition arises because the phase of the source (i.e. the nonlinear polarization wave) at a given point is determined by the derivative of the envelope of the laser pulse at that point. Therefore, the phase of the source stays synchronous with the phase of the THz wave when the laser envelope moves at the THz phase velocity. For GaSe, the phase matching angle can be determined as a function of the signal wavelength using the GaSe dispersion relation in V. G. Dimitriev, et al., “Handbook of Nonlinear Optical Crystals,” Springer, Heidelberg, 1999.